1. Field of the Invention
The present invention relates to a dual emission organic light emitting display device, and more particularly, to a dual emission organic light emitting display device that can display different images on a top surface and a bottom surface and/or a same image on the top surface and the bottom surface using a single pixel driver, and a method of driving the same.
2. Description of the Related Art
An organic light emitting display device is an emissive display in which light is created by electrically exciting organic compounds. That is, the organic light emitting display device includes an organic light emitting diode having an organic emission layer interposed between an anode and a cathode. Thus, holes supplied from the anode combine with electrons supplied from the cathode in the organic emission layer to form hole-electron pairs, i.e., excitons. When the excitons transition from an exited state to a ground state, energy is generated so that the organic light emitting diode emits light.
In general, the organic light emitting display device can be classified into a passive matrix type and an active matrix type depending on a method of driving N×M pixels that are arranged in a matrix shape.
The passive matrix type can be fabricated by arranging anodes and cathodes in a matrix shape on a display region.
By contrast, in the active matrix type, a thin film transistor (TFT) is disposed on each pixel of a display region. Thus, a constant amount of current can be supplied to an organic light emitting diode irrespective of the number of pixels so that the active matrix type can emit light with uniform luminance. Also, since the active matrix type consumes less power than the passive matrix type, it can be applied to high-resolution large-sized display devices.
Also, the organic light emitting display device can be categorized into a bottom-emitting type and a top-emitting type depending on the direction in which light generated from an organic emission layer is emitted.
In the bottom-emitting type, light generated from an organic emission layer is emitted toward a glass substrate. The bottom-emitting type includes a reflective layer (for a cathode) disposed on the organic emission layer and a transparent electrode (for an anode) disposed under the organic emission layer. Here, in the active matrix type, since a portion where a TFT is formed does not transmit light, the light emitting area can be reduced.
By contrast, in the top-emitting type, a transparent electrode (for a cathode) is disposed on an organic emission layer, and a reflective layer (for an anode) is disposed under the organic emission layer. Here, since light generated from the organic emission layer is emitted in a direction opposite to the substrate, the area or portion that that transmits light can increase, thereby increasing the luminance of the organic light emitting display device.
A dual emission organic light emitting display device is an organic light emitting display device in which a top-emitting type and a bottom-emitting type are formed on a single substrate.
Typically, the dual emission organic light emitting display devices can be categorized into first, second, and third types.
The first type employs a transmissive light emitting device structure so that the same image is displayed on both surfaces. However, since this first type displays the same image on both surfaces, it can only be applied to a few number of applications.
The second type includes two bottom-emitting substrates that are brought into contact with each other, so that different images are displayed on both surfaces. However, in the second type, although different images can be displayed on both surfaces, its production cost is high (doubled), and it is difficult to encapsulate such a device, thereby lowering its reliability.
In the third type, a single substrate is divided into a top emission region and a bottom emission region so that the top emission region and the bottom emission region can be separately driven. However, the third type allows an image to be only partially displayed on both surfaces.